Generally, a back of a human body is composed of a spinal joint of S-shape from a neck to a hip thereof. This is called as a backbone. The backbone includes a cervical vertebra, which is a neck part, a thoracic vertebra, which is a chest party, and a lumbar vertebra and a tailbone. Among them, the lumbar part is called as a waist. Here, the pain in the waist is called as a lumbar disc. Especially, the weight of the entire human body is applied the most on the waist in a mankind walling upright. Accordingly, a great number of people suffer from the lumbar disc.
In case of the lumbar disc, when he is bedridden, the pain is very weak owing to a small load. When he is standing, the pain is weak in the second place. In the meantime, when he is seated with his waist bent, the pain is most severe owing to a big load on the spine.
Accordingly, since it is forced to basically bring about the lumbar disc owing to the walking upright, a chair is one of the most convenient tools for human life.
The conventional chair 50, as shown in FIG. 6a, includes a seat plate part 51 for sitting thereon, a backrest 52 for supporting the waist, an armrest 53, and a chair leg 54. However, most of the conventional chair are provided with the backrest 53 for only simply support and protect the waist thereof during working and sitting on the chair. Since this conventional chair can concentrically support only the lumbar, it cannot protect a cervical vertebra or a thoracic vertebra etc.
In order to solve the above problem, as shown in FIG. 6b, besides the seat plate part 51 for sitting thereon, the backrest 52 for supporting the waist, the armrest 53, and the chair leg 54, an improved chair 60 including a thoracic support 55 and a cervical support 56 for sufficiently protect the thoracic vertebra and the cervical vertebra had been variously disclosed in shape and structure so as to effectively support the back portion and the neck portion.
However, in most of the conventional chairs 60, only the structures of the thoracic support 55 and the cervical support 56 thereof are variously and conveniently changed so as to protect only the thoracic vertebra and the cervical vertebra. That is, it cannot appropriately and flexibly cope with various sizes of the sitting height of the person sitting on the chair. In other words, where the persons having different sitting heights are sat on the same chair, it slows down the function capable of supporting and protecting the cervical vertebra or the thoracic vertebra etc. owing to the backrest that does not suit various body types. Hence, it can lead to waist health problems of the person working and sitting on the chair for a long time.